Technical Field
Embodiments of the subject matter disclosed herein generally relate to data processing for seismic surveying and, more particularly, to quality control data processing with respect to energy provided by a seismic source.
Discussion of the Background
Seismic exploration involves surveying subterranean geological formations, e.g., to locate hydrocarbon deposits in subterranean reservoirs. A survey typically involves deploying seismic source(s) and seismic sensors at predetermined locations. The sources generate seismic waves, which propagate into the geological formations. Subsurface features of the formations change the direction of propagation or other properties of incident seismic waves.
In reflection seismology, the seismic sensors detect reflections of the seismic waves off subsurface features or interfaces between features. The depth and the horizontal location of features causing reflections of seismic waves are estimated by measuring the time it takes for the seismic waves to travel from the seismic sources to the seismic sensors. Some seismic sensors are sensitive to pressure changes (e.g., hydrophones) and others are sensitive to particle motion (e.g., geophones, accelerometers). The sensors produce seismic data of the detected reflected seismic waves. Analysis of the seismic data can then indicate the presence or absence of probable locations of hydrocarbon deposits.
One type of seismic source is an impulsive energy source, such as dynamite for land surveys or a marine air gun for marine surveys. The impulsive energy source produces a relatively large amount of energy that is injected into the earth in a relatively short period of time. Another type of seismic source is a seismic vibrator, which is used in connection with a “vibroseis” survey. For a seismic survey that is conducted on land, the seismic vibrator imparts seismic waves into the earth at a relatively lower energy level than the signal that is generated by an impulsive energy source. However, the energy that is produced by the seismic vibrator lasts for a relatively longer period of time.
Excitation patterns for seismic vibrators are referred to herein as “pilot signals” and are generally designed before a seismic survey commences. Pilot signals are tuned (e.g., in duration and bandwidth) for specific predicted characteristics (e.g., moisture content) of the vibrator and the earth in the area being surveyed. However, the earth at the survey site may not have those characteristics. Differences between the actual and predicted characteristics can reduce the accuracy or usefulness of the survey data. Since surveys can be quite time-consuming, there is a need to determine as the survey progresses whether the data being collected are sufficiently accurate. This is referred to as “quality control” or “QC.” For example, it is desirable to compare distortion, phase and fundamental amplitude between the ground force (the force applied by the source to the ground) and the pilot signal, or in general between a source signal representing the action of the source and the pilot signal. Moreover, recent advances in vibroseis technology permit designing vibroseis sweeps with frequency down to 1 Hz and up to 300 Hz, as opposed to the 8-80 Hz range of former sweeps. There is a need for improved QC able to analyze ground forces over such a frequency range in a way that permits results that can be readily interpreted by field personnel. There is also a need for QC usable during low-frequency ramp-ups and high-frequency ramp-downs.
U.S. Patent Application Publication No. 2011/0182143 by Liu et al., published Jul. 28, 2011, the entire content of which is incorporated herein by reference, describes that traditional seismic data quality control involves applying a linear regression analysis to the seismic data for purposes of sorting out noisy or weak seismic traces from the remaining traces. A linear trend is determined in trace amplitude versus sensor offset. The linear trend is used to reveal a geophysical trend of the raw shot gather and allows traces to be judged as relatively weak or noisy based on this trend. In this manner, thresholds can be constructed above and below the determined trend for purposes of constructing a filter to reject the noisy and weak traces that fall outside of these thresholds. However, this scheme can itself produce noisy data that is difficult to interpret in the field. Other schemes involve comparing the ground force signal to the pilot signal in corresponding 0.5 s windows of each signal. Reference is also made to U.S. Pat. No. 6,148,264 to Houck et al., issued Nov. 14, 2000, the entire content of which is incorporated herein by reference.
However, all these methods do not produce good quality QC data at low and high frequencies. There is, therefore, a continuing need for ways of analyzing energy provided by a seismic source to determine, during a survey, whether the data are accurate.